Ultrastructural immunogold labelling of human Langerhans cells enriched epidermal cell suspension

HLA class I antigen (heavy and light chain) expression by Langerhans cells and keratinocytes of the normal human epidermis: ultrastructural quantitation using immunogold labelling procedure

Using an immunogold labelling procedure, we quantified the density of major histocompatibility (MHC) class I antigens on the surface of Langerhans cells (LCs) and keratinocytes of the normal human epidermis. According to ultrastructural features, keratinocytes were divided into three subpopulations: stratum basalis (SBK), stratum spinosum (SSK), and stratum granulosum keratinocytes (SGK), and analyzed separately. For this purpose, three monoclonal antibodies (MCAs) were employed: an anti-HLA A,B,C, and anti-B2-microglobulin (B2-m), and a polymorphic anti-HLA A2 Aw69 MCA. Under electron microscopy, quantitative analysis demonstrated: (a) the presence of a high amount of HLA monomorphic determinants on SBK and SSK and moderate but significant labelling of SGK; (b) the very weak density of MHC class I antigens on the surface of epidermal LCs; (c) the expression, at an identical level, of the HLA heavy chain common determinant (HLA A,B,C), B2-m, and the alloantigen HLA A2 by all epidermal cells (ECs) apart from SGKs and LCs that presented far fewer HLA A2 sites than monomorphic determinants (B2-m and HLA A,B,C); (d) the absence of HLA class I on corneocytes and a moderate labelling of melanocytes. A knowledge of the precise quantitative distribution of HLA class I antigens among various cell subpopulations of the normal human epidermis would be very useful for the study and follow-up of cutaneous malignancies that are known to lose these molecules as well as for the understanding of immune responses, especially allospecific, that involve the skin.

Human epidermal Langerhans cells internalize by receptor-mediated endocytosis T6 (CD1 "NA1/34") surface antigen. Birbeck granules are involved in the intracellular traffic of the T6 antigen

Using immunogold staining of a suspension of living human epidermal cells to identify the Langerhans cell membrane-associated antigen T6 (revealed by the monoclonal antibody BL6), we have observed internalization of T6 antigen in Langerhans cells. This phenomenon is at least partly due to receptor-mediated endocytosis involving coated pits, coated vesicles, endosomes, the smooth endoplasmic reticulum, and lysosomes. These ultrastructural results suggest that T6 antigen may be part of a receptor site. Following receptor-mediated endocytosis, the appearance in the cell center of the first labeled Birbeck granules suggests that Birbeck granules could represent T6 intracellular transport organelles carrying T6 from the central part of the cell to an unknown destination.

Growth and differentiation of human keratinocytes on extracellular matrix

Extracellular matrices (ECM) have been reported to enhance epithelial cell attachment and proliferation as well as to induce differentiation in vitro. Since ECM components are physiological constituents of the dermoepidermal basement membrane, we studied the growth and differentiation of human keratinocytes on ECM in order to determine the benefits of culturing epidermal epithelial cells (keratinocytes) on reconstituted basement membranes. Disaggregated epidermal cells were grown in primary and subcultures in liquid medium; the attachment of the cells was greatly enhanced by ECM and noted within the first few hours after seeding; cells formed small islets that reached confluence within 2-12 days depending upon the plating density and the type of culture (primary or passages). Histological and ultrastructural cross-sections of the cultures clearly indicated that a multilayered epithelium can be obtained including a basal cell layer, several intermediate cell layers with cytoplasmic organelles, intermediate size filaments, desmosomes, and keratohyaline granules, and an upper layer of anucleated cells. Using immunofluorescence, both pemphigus and pemphigoid (basal membrane zone) antigens were expressed. The keratin pattern noted indicated that these epithelia differentiate and keratinize but do not express a complete program of keratinization, a finding usually noted when cells are grown submersed. These data show that ECM favor epidermal cell proliferation and differentiation and suggest that they may be used to obtain large amounts of epidermal equivalent suitable for grafting and/or in vitro studies.

Human epidermal Langerhans cells cointernalize by receptor-mediated endocytosis "nonclassical" major histocompatibility complex class I molecules (T6 antigens) and class II molecules (HLA-DR antigens)

HLA-DR and T6 surface antigens are expressed only by Langerhans cells and indeterminate cells in normal human epidermis. We have previously demonstrated that T6 antigens are internalized in Langerhans cells and indeterminate cells by receptor-mediated endocytosis. This process is induced by the binding of BL6, a monoclonal antibody directed against T6 antigens. In the present study, using a monoclonal antibody directed against HLA-DR antigens, on human epidermal cells in suspension, we show that the surface HLA-DR antigens are also internalized by receptor-mediated endocytosis in Langerhans and indeterminate cells. Moreover, using immunogold double labeling, we demonstrate that T6 and HLA-DR antigens are internalized through common coated regions of the membrane of Langerhans or indeterminate cells. The receptor-mediated endocytosis that is induced involves coated pits and vesicles, receptosomes, lysosomes, and also, in Langerhans cells, the Birbeck granules. Thus, T6 antigens, which are considered to be "unusual" or "nonclassical" major histocompatibility complex class I molecules, and the major histocompatibility complex class II molecules, HLA-DR, are internalized in Langerhans and indeterminate cells through common receptor-mediated endocytosis organelles.

Epidermal cell-derived lymphocyte differentiating factor (ELDIF) inhibits in vitro lymphoproliferative responses and interleukin 2 production

We have examined the biologic characteristics and immunologic properties of epidermal cell-derived lymphocyte differentiating factor (ELDIF), a lymphocyte differentiating factor produced by cultured human keratinocytes. The ELDIF was semipurified by a gel filtration procedure. This factor, which is distinct from prostaglandins, epidermal cell-derived thymocyte activating factor (ETAF), and the well-known thymic hormones (thymulin, thymopoietin, and thymosin alpha 1) did not exhibit any interleukin (IL)-1, IL-2, or IL-3 activity. It strongly inhibited in vitro lymphoproliferative responses of normal mouse spleen cells to phytohemagglutinin, concanavalin A, and lipopolysaccharide. This dose-dependent phenomenon was associated with a suppression of IL-2 production rather than any toxic effect. It can be concluded that ELDIF, a product of human epidermal cells, which displays in vitro T-cell differentiation and regulatory activities, could be of major importance in vivo in the control of cutaneous inflammatory reactions.

Loss of allogeneic T-cell activating ability and Langerhans cell markers in human epidermal cell cultures

Human Langerhans cells are the only epidermal cells that express the T6 and HLA-DR antigens and are responsible for the in vitro allogeneic T-cell proliferative responses in the mixed skin cell lymphocyte reaction (MSLR). To investigate the presence of Langerhans cells in normal human epidermal cell cultures, epidermal cell suspensions obtained from normal human skin specimens and from the subsequent epidermal cell cultures were analyzed by indirect immunofluorescence for the presence of T6 and HLA-DR determinants. In parallel, MSLRs were conducted with suspensions of cultured epidermal cells as stimulatory cells. These studies present evidence that when human epidermal cells are grown in culture, they loose both the ability to stimulate the proliferation of allogeneic T lymphocytes in vitro and their expression of HLA-DR and T6 antigens. The T6 antigens were lost during the first 2 weeks of culture, while HLA-DR determinants were still expressed by a small number of cells and were progressively lost through duration of cultures. The loss of HLA-DR antigens closely paralleled the progressive inability of human epidermal cells in culture to stimulate allogenic T cells in MSLR.

Immunogold technique applied to simultaneous identification of T6 and HLA-DR antigens on Langerhans cells by electron microscopy

A double-labeling immunogold technique in electron microscopy and specific monoclonal antibodies to surface antigens of Langerhans cells (OKT6 and BL2) were applied to assess directly the coexpression of two cell surface antigens (T6 and HLA-DR antigens) in a heterogeneous epidermal cell suspension. Electron microscopic examination of double-labeled cells revealed that all Birbeck granule-containing Langerhans cells bound OKT6 and BL2. The preparation of markers with colloidal gold particles and the procedure for double labeling are described. Several problems related to the steric hindrance and current artifacts are illustrated by micrographs and also discussed.

Human epidermal cell-induced generation of alloreactive cytotoxic T-lymphocyte responses against epidermal cells. Requirement for DR-positive Langerhans cells

Human epidermal cells act as stimulators in the mixed-skin cell lymphocyte culture reaction (MSLR). To analyse the generation in MSLR of alloreactive cytotoxic T lymphocytes (CTL) in cell-mediated cytolysis of human epidermal cells. a phenomenon suggested by various observations of skin inflammatory processes in vivo, 18-h 51Cr-release assays against epidermal cells cultivated on collagen-coated plates (epidermal cells autologous to the stimulator cells in MSLR) were conducted after allogenic human MSLR. To analyse the role of human Langerhans cells and related epidermal dendritic cells, which are the only cells expressing the DR-Ia-like (class II) antigens in normal epidermis and in suspensions of normal epidermal cells, MSLR and CTL assays were conducted with, as stimulator cells, suspensions of normal human epidermal cells as controls, and, in parallel, suspensions of epidermal cells after preincubation with anti-class II monoclonal antibody and complement. The generation of alloreactive CTL to epidermal cells occurred only after allogenic MSLR and when targets and stimulator cells were from the same donor; it was abolished when epidermal cell suspensions used in MSLR were depleted in HLA-DR-expressing cells. These findings demonstrate that an epidermal cell-induced generation of cell-mediated cytotoxicity to human epidermal cells may occur in vitro. Langerhans cells and other class II antigen-expressing epidermal cells (dendritic indeterminate cells) are necessary for an optimal in vitro sensitization in MSLR and the subsequent generation of alloreactive CTL towards epidermal cells in man.

The semi-quantitative distribution of T4 and T6 surface antigens on human Langerhans cells

We have used indirect immunogold electron microscopy to compare the respective density of cell membrane determinants revealed by OKT6 and OKT4 monoclonal antibodies on normal human Langerhans cells (LC): 12.9 +/- 3.5 gold granules were noted per cell section on OKT4-positive LC whereas 236.8 +/- 23.5 granules were counted per cell section on OKT6-reactive cells. These results confirm that human LC react with OKT4 antibody and they demonstrate a marked quantitative difference on LC surface between the antigenic determinants recognized by OKT6 and OKT4 antibodies.

In vitro studies of epidermal antigen-presenting cells. The mixed skin lymphocyte reaction: an in vitro model for the generation of alloreactive cytotoxic T cells by human epidermal cells

Langerhans cells and indeterminate cells are the unique antigen-presenting epidermal cells participating in human lympho-epidermal interactions. They bear class II HLA-DR molecules, can substitute for macrophages in antigen presentation, induce a T-cell proliferative response to antigens and haptens in sensitized donors, and are necessary for alloantigen T-cell activation and generation of alloreactive cytotoxic T cells in vitro. Indirect immunofluorescence and immunogold electron microscopy on class II positive epidermal cell enriched suspensions (panning, FACS) indicated two populations of DR-positive epidermal cells: strongly DR-positive cells (25-30, 8% of positive epidermal cells) and faintly DR-positive cells, with a density of surface DR sites of respectively 5 X 10(5) and 1 X 10(5). Most Langerhans cells are among this second group while indeterminate cells are usually strongly DR-positive. OKT6-labelled cells were only typical Langerhans cells.

Detection of OKT6-positive cells (without visible Birbeck granules) in normal peripheral blood

The specificity of a monoclonal antibody (OKT6) for peripheral blood mononuclear cells was examined by indirect immunofluorescence and ultrastructural immunogold labelling. Some rare peripheral blood mononuclear cells (approximately 1%) expressed T6 antigen on their membrane surface, as determined by light microscopy and cytofluorometry. Electron microscopic examination of immunogold-labelled cells revealed that OKT6-positive cells were dendritic, lacking the Birbeck granules and expressed variable density of the membrane T6 antigen. The relationship of such cells with Langerhans' cells is discussed.